The stock of America's largest chain of drive-in restaurants revved up on positive earnings news. Here's why it can keep on rolling.

After the close of trading Monday, Sonic (ticker: SONC) reported earnings of $3.6 million, or six cents a share, up from three cents a share in the year-earlier period. Excluding one-time items, adjusted earnings were five cents a share for the quarter ended Feb. 28. Revenue fell 3.4% to $111.1 million. Analysts were...